As a structure for attaching turbine impeller such as steam turbine to a rotary shaft, there is a structure as shown in FIG. 14 in which a blade 14 of the turbine impeller is fitted in each of a multiplicity of tree-shaped grooves 12 that are provided in a periphery of the rotary shaft 10. FIG. 15 is a cross-sectional view showing one of the tree-shaped grooves 12 in enlargement. Each of the tree-shaped grooves 12 is symmetrical with respect to a groove center S. A width of the groove 12 is gradually increased and reduced, and generally reduced in a direction of groove depth (i.e., downward direction as seen in FIG. 15), like an inverted christmas-tree. In each of side surfaces 16a, 16b, a plurality of recessed portions 18 and a plurality of protruding portions 20 are provided to be alternately arranged.
There is proposed a tool for cutting the tree-shaped grooves 12 as shown in FIG. 15. A formed rotary cutting tool described in Patent Document 1 is an example of such a tool. According to this technique, a cutting edge diameter defined by cutting edges is increased and reduced, and is generally reduced in an axial direction toward a distal end of the tool. The formed rotary cutting tool is moved relative to a workpiece in a direction perpendicular to an axis of the cutting tool while being rotated about the axis, for cutting a groove having a predetermined shape, so that the inverted christmas-tree shaped groove 12 as shown in FIG. 15 can be cut in a simple and easy manner. Such a formed rotary cutting tool is called commonly as a christmas-tree cutter.